1. Field of the Invention
The present invention generally relates to high precision imaging using a reflective optical element and, more particularly, to high precision lithography exposure systems and methods using one or more adaptive, reflective optical elements to minimize aberrations and measurement and control therefor which is controlled by a combination of passive and active actuators.
2. Description of the Prior Art
Many manufacturing and scientific processes require the use of optical systems having extremely high accuracy and precision and freedom from aberrations as well as the ability to make observations and/or exposures in wavelength regimes well outside the visible spectrum. For example, at least one lithographic exposure process is invariably required for establishing the location and basic dimensions of respective electrical or electronic elements in semiconductor integrated circuits in which the number of such elements on a single chip can extend into the tens if not hundreds of millions. The respective electrical or electronic elements can be very small and placement in close proximity, sometimes referred to as high integration density, is highly desirable in order to reduce signal propagation time and susceptibility to noise as well as to achieve other advantages such as increased functionality and, in some cases, manufacturing economy. These circumstances provide strong incentives to develop smaller minimum feature size regimes which must be established through lithographic exposures of a resist. Therefore, resolution and aberration of the exposure must be held within a very closely defined budget which is a small fraction of the minimum feature size.
The resolution of any optical system is a function of the wavelength of the energy used for the exposure although some arrangements such as phase-shift masks have allowed exposure resolution to be extended below the wavelength of the exposure radiation. Nevertheless, resolution of extremely small features requires correspondingly short wavelengths of radiation. Accordingly, use of X-rays for lithographic exposure are known but not widely used due to the requirement for fabrication of an exposure mask at the same minimum feature size as the final desired pattern since reduction of the size of the pattern cannot be achieved with X-rays. Optical and electron beam projection systems, however, can achieve such image pattern size reduction in the exposure pattern relative to feature sizes in a reticle which establishes the pattern to be exposed. However, between these two techniques, reticles for electron beam projection are generally far more expensive than optical reticles and, perhaps more importantly, require many more exposures to form a complete integrated circuit pattern since the exposure field at the chip is comparatively more limited in electron beam projection systems. Therefore, there is substantial continued interest in optical lithographic exposure systems and extending their capabilities to shorter wavelengths, such as extreme ultraviolet (EUV).
EUV wavelengths are generally considered to be in the range of about 12 to 14 nanometers and more specifically within a range of less than one nanometer in a band centered on approximately 13 nanometers. At such wavelengths, most imaging materials which are transparent in the visible spectrum and which are suitable for lenses are substantially opaque to the imaging radiation. Therefore, optical systems have been developed and are known which have only reflective elements. Such fully reflective systems are usually more complex than lens systems since interference between illumination of the reticle and illumination of the target with the projected pattern must be avoided. While the number of reflective elements in such lens systems are generally fewer than the number of elements in comparable refractive lens systems, reflective surfaces are much more sensitive to surface aberrations and the freedom from aberrations maintained or well-corrected throughout the entire optical system. The maintenance of high manufacturing yield in the above-discussed exemplary environment thus requires not only high stability of the optical system but frequent measurement and adjustment to assure an adequately high level of optical performance of the system.
While techniques of measurement of wave-front aberrations are well-known and sufficient to accurately characterize the performance of optical systems and elements thereof, practical arrangements for conducting such measurements are difficult and complex. For instance, measurements cannot be made on axis or within the exposure/projection field during an exposure without interference with that exposure (e.g. by casting shadows or otherwise occupying a portion of the focal plane of the system where the target is located). Measurements performed between exposures cannot be regarded as measurements of optical performance during the exposure, itself, and do not directly characterize the lithographic image, but are often the only practical solution at the current state of the art even though sources of error may be introduced. Optical performance generally degrades with increasing distance from the optical axis of the system and, as a practical matter, it is desirable to use as much of the field where sufficient precision, resolution and freedom from aberrations can be maintained for projection of the desired image; generally precluding such measurements which, in any event, may not directly or even predictably correspond to the on-axis performance of the element or system.
Active optics are known but have not been widely used to date. Active optics involve the ability to change the overall or local shape of optical elements to alter the optical properties of the element. The article “Active Optics: A New Technology for the Control of Light” by John W. Hardy, Proc. of the IEEE, Vol 66, No. 6, June, 1978, provides an overview of this technology and is hereby fully incorporated by reference. In particular, some general suggestions are made for provision of mechanical arrangements for achieving localized or generalized deformations of reflecting optical elements to achieve different optical effects such as compensating for atmospheric turbulence. Nevertheless, measurement to achieve any particular optical effect remains extremely complex and difficult as discussed therein and the deformation of optical elements is limited and difficult to control, particularly when it is considered that deformations can be comprised of multiple components which may be relatively difficult to distinguish and which may take many different forms which are difficult to characterize. For example, some relatively large components of deformation of an optical element may be caused by manufacturing variations and/or mounting arrangements while some relatively smaller and generally more localized components of deformation may be due to thermal effects including but not limited to irregular absorption of radiation in accordance with the projected pattern. Further, it should be recognized that the corrections needed may be substantially less than the wavelength of imaging radiation, which, itself, may be very short to achieve adequate resolution but may be as much as several orders of magnitude larger.
Additionally, localized corrections may be necessary for relatively small areas of the optical element, particularly to compensate for higher-order aberrations which generally tend to be an incident of manufacturing variations and/or mounting imperfections but may correspond to localized thermal heating from absorption of patterned exposure radiation. Further, it is generally desirable for the optical elements to be relatively stiff and thick to provide significant thermal mass and stability; requiring significant power or force to achieve localized shape correction while the size of actuators must be correspondingly limited although the force available therefrom must be relatively large. The actuators themselves may thus become a localized heat source that complicates correction particularly as the numbers thereof are increased in order to provide correction of aberrations through high order aberrations.